With soil temperatures 2oC colder and air 0.5oC colder than average, nurseries have had to re-think their usual growing practices to ensure this year’s show is as colourful as always.
Apple trees normally seen flowering in April are currently in full bloom in the Homebase Garden, designed by Adam Frost.
RHS chief horticultural advisor Guy Barter said the growing season was about a fortnight behind.
"There's been a delay in some of the early summer colour, but with a bit of warm weather and sunshine, it won't be long before the summer-flowering plants burst into bloom."
Among this year’s May belles are highly scented, old-fashioned grandiflora sweet peas, shown by Staffordshire nursery Eagle Sweet Peas. Lord Nelson, Cupani and Queen Alexandra are some of the Lathyrus odoratuscultivars which will be on display.
Hardy's Cottage Garden Plants of Hampshire, which has won 17 Gold medals at the RHS Chelsea Flower Show, will bring a display of herbaceous perennials, including Lamprocapnos (bleeding hearts), Lamium orvala (deadnettle) and Paeonia emodi.
Rhododendrons are back in fashion this year, featuring in Delancy's East Village Garden and Kate Gould's The Wasteland.
The show runs until Saturday.